The present invention relates to aqueous coating compositions. In particular, the aqueous coating compositions of the present invention have a low level of volatile organic compounds (xe2x80x9cVOCxe2x80x9d) and provide dry films having excellent resistance properties and durability. Aqueous coating compositions of the present invention are useful in polish and coating applications where durability and resistance properties are important.
Various methods have previously been employed to prepare durable, resistant aqueous coating compositions. One approach has been to prepare zinc-complexed polymers. Another approach uses polymers having a glass transition temperature (xe2x80x9cTgxe2x80x9d) above ambient temperature in conjunction with volatile organic coalescents to aid in film formation. Higher Tg polymers provide better durability and resistance properties, but also require higher levels of VOC. Unfortunately, coating compositions having either zinc or high levels of VOC present regulatory and environmental concerns.
One attempt to overcome the problems associated in preparing a durable, environmentally friendly coating with good resistance properties is disclosed in U.S. Pat. No. 5,428,107. The disclosed coating composition contains a polymer having acetoacetate groups and acid-functional monomer. The polymer is post-treated with amino-functional silane. In one embodiment of U.S. Pat. No. 5,428,107, the acid-functionality on the polymer can be further reacted with divalent metal ions. While this composition can be used to produce durable, zinc-free coatings, it still contains relatively high levels of VOC. Additionally, coating compositions containing amino-functional silane are more expensive than conventional coatings.
The present invention has overcome the environmental problems associated with zinc and high VOC, in the absence of expensive amino-functional silanes. We have found that durable, resistant coatings can be formed from aqueous compositions having VOC levels below seven percent by formulating an acetoacetate-functional polymer with selected metal ions. The coating compositions of the present invention can be used to produce tough, resistant coatings on various substrates such as floors, walls, wood, metal, plastic, stone, paper, leather and concrete.
In a first aspect of the present invention, there is provided a coating composition comprising:
a) polymer comprising, as polymerized units, from 0.5 percent to 100 percent by weight acetoacetate-functional monomer; wherein the polymer has a glass transition temperature in the range of from xe2x88x9220xc2x0 C. to 150xc2x0 C.; and
b) divalent metal ion;
wherein the molar ratio of acetoacetate-functional monomer to divalent metal ion is in the range of from 20:1 to 2:1; wherein the composition is substantially free of sulfopolyester; and wherein the composition is substantially free of amino-functional silane.
In a second aspect of the present invention, there is provided a process comprising:
a) forming a coating composition by admixing:
1) polymer comprising, as polymerized units, from 0.5 to 100 percent acetoacetate-functional monomer; wherein the polymer has a glass transition temperature in the range of from xe2x88x9220xc2x0 C. to 150xc2x0 C.; and
2) divalent metal ion;
wherein the molar ratio of acetoacetate-functional monomer to divalent metal ion is in the range of from 20:1 to 2:1; wherein the composition is substantially free of sulfopolyester; and wherein the composition is substantially free of amino-functional silane;
b) applying said coating composition to a substrate to form a coated substrate; and
c) drying said coated substrate.
In a third aspect of the present invention, there is provided a coating composition comprising:
a) polymer comprising, as polymerized units, from 0.5 percent to 100 percent by weight acetoacetate-functional monomer; wherein said polymer has a glass transition temperature in the range of from xe2x88x9220xc2x0 C. to 150xc2x0 C.; and wherein said polymer comprises less than 5 percent by weight acid-functional monomer;
b) divalent metal ion; and
c) amino-functional silane;
wherein the molar ratio of acetoacetate-functional monomer to divalent metal ion is in the range of from 20:1 to 2:1; and wherein the molar ratio of acetoacetate functional monomer to amino-functional silane is in the range of from 20:1 to 2:1.
In a fourth aspect of the present invention, there is provided an article comprising a substrate coated with the aqueous coating composition of the present invention.
Polymers useful in the present invention have, as polymerized units, acetoacetate-functional monomer. These polymers can be homopolymers, copolymers or mixtures of such polymers. Suitable acetoacetate-functional monomers include acetoacetoxyethyl acrylate, acetoacetoxypropyl methacrylate, acetoacetoxyethyl methacrylate, allyl acetoacetate, acetoacetoxybutyl methacrylate, and 2,3-di(acetoacetoxy)propyl methacrylate. A preferred monomer is acetoacetoxyethyl methacrylate (xe2x80x9cAAEMxe2x80x9d).
The acetoacetate-functional polymer may contain from 0.5 percent to 100 percent by weight of the acetoacetate-functional monomer. The amount of acetoacetate-functional monomer required will vary depending upon the end-use application. Generally, the acetoacetate-functional monomer level will be between 1 percent and 75 percent by weight. Conventional floor polish and coatings polymers will usually contain from 0.5 percent to 50 percent by weight acetoacetate-functional monomer. Polymers having a molecular weight of from 1,000 to over one million are useful in the present invention. In general, lower molecular weight polymers will have higher relative levels of acetoacetate-functional monomer. For example, a copolymer having a molecular weight under 10,000 would typically contain 30 percent or more of acetoacetate-functional monomer.
The polymers of this invention are most often copolymers of the acetoacetate-functional monomer and other monomers. As used herein, xe2x80x9c(meth)acrylatexe2x80x9d is used to mean either acrylate or methacrylate. Examples of useful comonomers include simple olefins such as ethylene, alkyl (meth)acrylates where the alkyl group has 1 to 20 carbon atoms (preferably 1 to 12 carbon atoms), vinyl acetate, acrylonitrile, styrene, isobornyl methacrylate, acrylamide, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, N-vinyl pyrolidinone, butadiene, isoprene, vinyl halides such as vinyl chloride and vinylidene chloride, alkyl maleates, and alkyl fumarates.
In a preferred embodiment of the present invention, the polymer also contains, as polymerized units, acid-functional monomers or salts thereof Suitable acid-functional monomers include, for example, carboxylic acid monomers, sodium vinyl sulfonate, sodium methallyl sulfonate, phosphoethylmethacrylate, or 2-acrylamido-2-methylpropanesulfonic acid. Preferably, the acid-functional monomer is a carboxylic acid monomer such as, for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid, and itaconic acid. Acid-functional monomers are incorporated at a level sufficient to provide removability of a dried film formed from the composition. The acid-functional monomer level is preferably greater than 3 percent and more preferably from 5 to 75 percent by weight of the polymer. Most preferably, the acid-functional monomer level is in the range of from 8 percent to 50 percent by weight of the polymer, to provide detergent resistance and water resistance to the dried coating. If too much acid-functional monomer is incorporated into the polymer, the resistance of the film to scrubbing with alkaline detergent solutions and resistance to aqueous solutions are greatly compromised.
Also, acetoacetate-functional polymers having no acid-functional monomer are useful in the present invention. When used in the composition of the present invention, such polymers produce durable, resistant coatings useful in applications that do not require film removal, such as those used to seal floors and furniture.
In another embodiment of the invention, where the level of acid-functional monomer in the acetoacetate-functional polymer is below five percent by weight of the polymer, the polymer may be reacted with amino-functional silane. The amino-functional silane is added by post reaction of an effective amount of aminosilane with the acetoacetate-functional monomer in the polymer.
In another embodiment, polymers of this invention may be designed to swell in the presence of acidic stripper solutions, providing a mode for removability of the dried film from surfaces such as, for example, floors. Removability can be achieved by incorporating amino-functional monomer into the polymers useful in this invention. The polymers preferably contain from 3% to 30% amino-functional monomer, based on the total weight of monomers. More preferably, the amount of amino-functional monomer is from about 5% to about 20%, based on the total weight of monomers. Examples of amino-functional monomers are dimethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate, methylaminoethylacrylate, and mixtures thereof.
It is also possible, and sometimes desirable, to include low levels of divinyl or polyvinyl monomers such as glycol polyacrylates, allyl methacrylate, and divinyl benzene, to introduce a controlled amount of crosslinking in the polymer. Additionally, one may wish to include conventional chain transfer agents to control molecular weight of the polymer, such as, for example, a mercaptan.
Generally, polymers useful in the present invention are prepared by means known in the art such as dispersion or emulsion polymerization in water. Preferably, the polymer is prepared by aqueous emulsion polymerization using a suitable free radical initiator and appropriate heating. Conventional dispersants may be used at levels in the range of from 0.1 percent to 6 percent by weight based on the weight of total monomer. Initiation can be either by thermal or redox initiation using conventional free radical initiators such as, for example, hydrogen peroxide, organic hydroperoxides, organic peroxides and inorganic peroxides at levels of from 0.05 percent to 3.0 percent by weight based on the weight of total monomer.
The invention may also be practiced using a water-soluble polymer. Water-soluble polymers are advantageously used as blends with conventional latex polymers, preferably those containing acetoacetate functional monomers. The blend of alkali-soluble resin and latex polymer has a particularly advantageous property combination of gloss and rheology and is useful in coatings and printing ink applications.
In another aspect of the present invention, the polymer is prepared by a multistaged emulsion addition polymerization process, in which at least two stages differing in composition are formed in sequential fashion. Such a process usually results in the formation of at least two mutually incompatible polymer compositions, thereby resulting in the formation of at least two phases. The mutual incompatibility of two polymer compositions and the resultant multiphase structure of the polymer particles may be determined in various ways known in the art. The use of scanning electron microscopy using staining techniques to emphasize the difference between the appearance of the phases, for example, is such a technique. Two phase polymers are particularly useful in coating compositions where faster solvent release is desired. The use of two phase polymers in a coating composition also allows for the use of lower levels of VOC.
The coating composition of the present invention is generally film-forming. xe2x80x9cFilm-formingxe2x80x9d, as used herein, means that the coating composition has a minimum Film Forming Temperature (xe2x80x9cMFFTxe2x80x9d) at, or below, the ambient temperature, to allow for fusion of the polymer into a continuous film. Volatile organic compounds, such as coalescents, can be used to temporarily lower the MFFT of a coating composition, allowing the polymer to form a film at a temperature below the Tg of that polymer. By xe2x80x9cvolatile organic compoundsxe2x80x9d or xe2x80x9cVOCxe2x80x9d, as used herein, is meant organic compounds having a boiling point at atmospheric pressure of less than 250xc2x0 C. Polymers of the present invention have a Tg in the range of from xe2x88x9220xc2x0 C. to 150xc2x0 C. and preferably from 0xc2x0 C. to 150xc2x0 C. If soluble polymers are used in the film-formation process, polymers of higher glass transition temperature are readily used since they are film-forming.
Polymers containing acetoacetate-functional monomers are prone to hydrolysis in water, particularly on heat aging. The hydrolysis occurs at nearly any pH and yields acetoacetic acid. This problem may be overcome by treating the acetoacetate-functional polymer with one molar equivalent of ammonia or a primary amine to form the enamine. The enamine is typically stable to hydrolysis at pH""s typically greater than 7.
The coating composition of the present invention contains divalent metal ion in addition to the acetoacetate-functional polymer. Divalent metal ions useful in this invention include, for example, zinc, calcium, magnesium, zirconium and mixtures thereof. Preferably, the coating composition is free of zinc. More preferably, the divalent metal ion used in this invention is an alkaline earth metal ion. Calcium, magnesium, or mixtures thereof, are particularly preferred. Coating compositions having alkaline earth metal ion provide a harder coating surface than compositions containing zinc. Thus, a lower Tg polymer may be used with alkaline earth metal ion, resulting in a lower use level of VOC coalescent in the coating composition. xe2x80x9cLow VOCxe2x80x9d, as used herein, means a total level of volatile organic compounds in the coating composition of less than 15%, preferably less than 9%, and most preferably less than 5% by weight based on the total coating composition.
Divalent metal ion modified polymers of this invention are prepared by adding to the coating composition an effective amount of divalent metal ion capable of reacting with the acetoacetate-functional polymer. The level of divalent metal ion is a function of the acetoacetate-functional monomer content of the polymer. Divalent metal ion levels useful in this invention are in the range of from 0.05 to 0.5 moles of divalent metal ion for each mole of acetoacetate functional monomer. This provides a molar ratio of acetoacetate-functional monomer to divalent metal ion in the range of from 20:1 to 2:1. In terms of equivalents, this is the same as two equivalents of acetoacetate-functional monomer to 0.05 to 0.5 equivalents of metal ion, or a ratio of equivalents of acetoacetate-functional monomer to divalent metal ion in the range of from 10:1 to 1:1. Preferably, the molar ratio of acetoacetate-functional monomer to divalent metal ion is in the range of from 10:1 to 2:1. In one embodiment of the present invention, the polymer may also contain acid-functional monomer. Since divalent metal ion can crosslink with both the acetoacetate-functional monomer and acid-functional monomer in the polymer, the useful level of divalent metal ion is a function of the sum of the acetoacetate-functional monomer and the acid-functional monomer.
If insufficient divalent metal ion is used in relation to the acetoacetate-functional polymer, properties such as, for example, black heel and scuff mark resistance, and mar resistance, of the dried coating may be compromised. Whereas, on the other hand, if more than 0.5 mole of divalent metal ion are used for each mole of acetoacetate-functional monomer, coating properties may become impaired.
The divalent metal ion is preferably introduced as a metal oxide, tetra-amino metal bicarbonate complex, a metal complex of an xe2x80x94NHxe2x80x94 or NH2xe2x80x94 functional compound or a metal salt. Preferably, hydroxides of an alkaline earth metal ion are used, such as, for example, calcium hydroxide and magnesium hydroxide.
Divalent metal ion modified coating compositions of the present invention are prepared by adding a specific quantity of divalent metal ion to the acetoacetate-functional polymer. The quantity of divalent metal ion added should be in specific proportion, for reasons stated earlier, to the acetoacetate-functional monomer content of the polymer, or, where applicable, the acetoacetate-functional monomer content plus acid-functional monomer of the polymer. The divalent metal ion is preferably added to the coating composition after the polymerization of the polymer. The preferred coating composition is a single package, stable composition.
In general, the divalent metal ion can be added directly to the acetoacetate-functional polymer. However, for optimum performance, processing of the final divalent metal ion modified polymer and shelf-life stability, an auxiliary surfactant may be required. This is particularly true in cases where the polymer is produced by emulsion polymerization. The auxiliary surfactant can be added before or after the addition of the divalent metal ion.
Surfactants may be characterized by their xe2x80x9cHydrophilic-Lipophilic Balancexe2x80x9d (xe2x80x9cHLBxe2x80x9d) value. Surfactants with HLB values of less than 10 are considered to possess more lipophilic character, while surfactants with HLB values greater than 10 are considered to possess more hydrophilic character. The preferred surfactants of this invention are non-ionic surfactants having HLB values greater than 10. More preferably, the HLB value is greater than 15.
Surfactant levels from 0 up to 10 percent by weight of the polymer can be used. The preferable level of surfactant is between 3 percent and 6 percent of the weight of the polymer. Surfactants useful in the practice of the present invention are, for example, non-ionic surfactants, such as octylphenoxypolyethoxyethanols, nonylphenoxypolyethoxyethanols, polypropyloxyethoxy alcohols, and ionic surfactants such as sodium lauryl sulfate and sodium stearate.
The coating composition of this invention containing an acetoacetate-functional polymer modified with divalent metal ion may be formulated for the chosen coating end use. Additives such as thickeners, dispersants, pigment, extenders, fillers, anti-freeze agents, plasticizers, adhesion promoters, coalescents, wetting agents, defoamers, colorants, biocides, soaps and slip agents may be optionally added to the composition.
The present invention provides for coating compositions that can be used to generate surface coatings for a variety of vertical and horizontal surfaces. More particularly, coating compositions of the present invention are useful as polishes, inks and other coatings. Dried films formed from the coating compositions exhibit improved properties such as black heel and scuff mark resistance, print resistance, mar resistance, block resistance, and impact resistance.
Substrates to which the coating composition of this invention may be applied include, for example, architectural substrates such as resilient and non-resilient floors, walls, marble, stone, terrazzo, concrete, asphalt, roofing substrates, linoleum; and industrial materials such as wood, particle board, medium density fiber board (MDF), metal, ceramics, leather, plastic, glass, paper, cardboard, kitchen cabinets and counter tops, and furniture products. Preferred substrates include furniture, flooring and paper. The coating composition of this invention may be applied to a substrate by methods well known in the art of applying coatings such as, for example, spray, brush, mop, roller and direct and reverse roll coating.
In some applications of this invention, the coated substrate is cured by heating up to 100xc2x0 C. Preferably, the coating composition of this invention is cured at ambient conditions of temperature, relative humidity and air velocity, the temperature being greater than 0xc2x0 C., in that the composition is formulated in an aqueous composition, and less than 40xc2x0 C.
This test is based on striking the coating at a shallow angle with a hard object. In the examples provided, the object was the fingernail of the individual performing the test. This test gives an indication of how the coating will resist marring, which leads to gloss reduction of the coating.
Two different procedures for preparing samples to be used in the mar resistance test are embodied in the following Examples. In Examples 1-20 and 25-32, a 1 mil (0.025 millimeter) thick film of the coating composition was drawn down on an vinyl composition tile and cured at ambient temperature for 24 hours. In Examples 21-24, an 8 mil (0.2 millimeter) thick film of the coating composition was drawn down on an aluminum panel and cured in a 65xc2x0 C. oven for 10 minutes, then removed and allowed to cool at ambient temperature and humidity for seven days. The cured samples formed by either procedure were then struck several times across the coating surface with the operator""s fingernail. The operator""s fingernail was kept parallel to the coated surface, and the impact angle was greater than 45 degrees from the normal of the surface, to increase the likelihood of marking the coating.
When comparing coatings, it is important that the same operator perform the test. This test is designed to distinguish relative differences.
The degree of damage to the coating surface was rated on a scale of from 1 to 5, as follows:
1=Coating can""t be visibly scratched
2=Very slight scratch, visible at only at a few angles
3=Slight scratch, visible at any angle
4=Very visible scratch
5=Coating is easily torn
The method for determining black heel and scuff resistance is described in Chemical Specialty Manufacturers Association Bulletin No. 9-73, except that commercially available rubber shoe heels were used in place of the recommended 2 inch (5.08 centimeter) rubber cubes.
Furthermore, instead of subjectively rating the coated substrate, we determined the number of marks per square inch (6.45 square centimeters) of the coated substrate area which was covered by black heel and scuff marks. Black heel marks are an actual deposition of rubber onto or into the coating.
A scuff mark, on the other hand, results from a physical displacement of the coating and appears as an area of reduced gloss. Scuff and black heel marks can occur simultaneously at the point where the heel impacts the substrate i.e., upon removal of a black heel mark, a scuff may be present.
Stackability is a measure of the block resistance of a coating. A 2 inch (5.08 centimeter) wide, 8 mil (0.2 millimeter) thick film of the coating composition was drawn down on two panels of pine veneer plywood which had been spray-coated with sealer. The panels were cured in a 65xc2x0 C. oven for 10 minutes then removed and allowed to cool for 5 minutes to approximately 30xc2x0 C. The panels were stacked crosswise, face-to-face and a 5 kilogram weight placed on top of the panels. The panels with weight were allowed to stand overnight, about eighteen hours, at ambient temperature. The weight was removed, and the panels were rated on the following scale based on ease of separation:
1=panels are glued together
2=panels are difficult to take apart
3=panels are relatively easy to take apart
4=panels are very easy to take apart
5=panel fall apart
VOC
The level of volatile organic compounds reported is a percentage by weight of organic coalescent that was added to the coating composition.
All polymer Tg values in these examples were measured by differential scanning calorimetry (DSC), used at a rate of heating of 10xc2x0 C. per minute with the Tg taken at the midpoint of the transition.